Permanently excited electrical machine

ABSTRACT

An electrical machine, in particular a direct current motor for vehicles, has a multi-pole stator which has an annular pole housing ( 1 ) and a plurality of magnets ( 2 ) that are located on the inside face of the pole housing ( 1 ). The electrical machine further includes a magnet splinter guard ( 3 ), which shields the magnets ( 2 ) inward in the radial direction toward the rotor, in which the magnet splinter guard ( 3 ) is formed from a rectangular blank, has an overlapping region ( 4 ), extending in the circumferential direction over the axial length of the magnet splinter guard ( 3 ). On each of the ends ( 6, 7 ) of the magnet splinter guard ( 3 ) located in the axial direction, a respective centering ring ( 8 ) is located, for centering the magnet splinter guard ( 3 ).

CROSS-REFERENCE

The invention described and claimed herein below is also described inPCT/DE 03/03393, filed on Oct. 13, 2003 and DE 103 02 454.9, filed Jan.23, 2003. This German Patent Application, whose subject matter isincorporated here by reference, provides the basis for a claim ofpriority of invention under 35 U.S.C. 119(a)–(d).

BACKGROUND OF THE INVENTION

The present invention relates to a permanently excited electricalmachine, in particular a direct current motor for vehicles.

From German Patent Disclosure DE 1160080 A1, a direct current motor isknown in which the stator poles are formed of rectangular permanentmagnets and pole shoes mounted on the permanent magnets. The permanentmagnets are secured to a pole housing by means of adhesive bonding. Theworking air gap between the stator and rotor is defined by the poleshoes.

In the known direct current machines, for various reasons, such as arough operating environment of a vehicle, material stresses in thepermanent magnet cannot reliably be prevented from causing cracks anddiscontinuities, which can cause small splinters or pieces of materialto become detached from the permanent magnet. If such material splintersget into the working air gap, this can cause seizing or blocking of therotor. This is critical, especially if the motor is used forpower-assisted steering in vehicles. It has therefore been proposed thata cylindrical magnet splinter guard be provided between the magnets ofthe stator and the rotor, the magnet splinter guard being retained onthe stator by means of two covering rings.

SUMMARY OF THE INVENTION

The electrical machine of the invention has the advantage over the priorart that has an inexpensive magnet splinter guard that is simple toproduce. The magnet splinter guard is formed of a rectangular blank andhas an overlapping region extending over the axial length of thesplinter guard. The result is a magnet splinter guard that is simple toproduce and that prevents pieces of material that splinter off from themagnet from getting into the working air gap between the stator and therotor. The overlap assures that splintered-off pieces cannot get inbetween the two layers of the overlapping region to reach the workingair gap. According to the invention, centering rings are also provided,which are located on both ends, in the axial direction, of the magnetsplinter guard. As a result, centering of the magnet splinter guard canbe made possible, so that a working air gap that is constant over thelength and is as narrow as possible is preserved. Thus the electricalmachine of the invention can be used especially in vehicles, forinstance in steering gears for power-assisted steering or as an electricmotor for introducing braking force, in which high functional safetymust be assured in order to prevent a vehicle from becoming unsteerable.In particular, the overload couplings presently used for preventing suchan unsteerable situation of the vehicle can be dispensed with.

By the provisions recited in the further claims, advantageousrefinements of or improvements to the electrical machine of theinvention are possible.

Especially preferably, the magnet splinter guard is embodied such thatin the installed state, it automatically presses radially outwardagainst the magnets and thus stays in position by its intrinsicelasticity, without further aids. The centering rings act as anadditional safety device.

To enable fast, simple centering that is automatically maintained uponinstallation, the centering rings each have a tapering region.

Also preferably, the centering rings enclose the magnets between thepole housing and the magnet splinter guard, so that there is nopossibility for magnet splinters to escape from this closed space.

To furnish improved magnetic flux, the magnets preferably have polelifts. The overlapping region of the magnet splinter guard is especiallypreferably located on these pole lifts.

Also preferably, a clamping strip is located on the outer circumferenceof the magnet splinter guard, to attain a fastening of the magnetsplinter guard by clamping of the clamping strip between two adjacentmagnets. The clamping strip also secures the magnet splinter guardagainst twisting.

To make especially simple installation possible, the axial ends of themagnet splinter guard are preferably bent slightly radially outward.

The overlapping region of the magnet splinter guard is also preferablyformed such that a radially outward-oriented graduation is embodied atthe overlapping region. As a result, the overlap can be designed suchthat the magnet splinter guard has a constant inside diameter, so thatthe air gap between the stator and the rotor is constant.

In order, with maximum certainty, to prevent unintentional loosening,the overlapping region of the magnet splinter guard is preferablyconnected in captive fashion. This can be accomplished for instance bymeans of gluing or welding or by interlocking of the overlappingregions, or the like.

Preferably, the tapering region of the centering rings is embodied as acone or as an outward-bulging region, or as an inward-bulging region, oras a stepped tapering region.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail in the ensuing descriptionin terms of an exemplary embodiment shown in the drawing. Shown are:

FIG. 1, a schematic perspective view of a magnet splinter guard in afirst exemplary embodiment of the present invention;

FIG. 2, a schematic sectional view of the magnet splinter guard shown inFIG. 1;

FIG. 3, a schematic sectional view of a magnet splinter guard in asecond exemplary embodiment of the present invention; and

FIGS. 4 through 7, schematic sectional views of various embodiments ofcentering rings according to the invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

In FIGS. 1 and 2, a magnet splinter guard 3 is shown in a firstexemplary embodiment of the present invention. As can be seenparticularly from FIG. 1, the magnet splinter guard 3 is embodiedsubstantially cylindrically, and on its outer circumference it has aclamping strip 5. The magnet splinter guard 3 is made from a rectangularblank and has an overlapping region 4, which is embodied in overlappingor covering fashion in the circumference direction of the magnetsplinter guard 3 and extends over the entire axial length of the magnetsplinter guard 3. The overlapping region 4 has a predeterminedoverlapping height H, so that over a certain circumferential length acontact of the two butt ends of the magnet splinter guard can beattained.

In the installed state shown in FIG. 2, the clamping strip 5 of themagnet splinter guard 3 is located between two permanent magnets 2, 2.The clamping strip 5 is embodied with a V-shaped cross section, and itclamps between the two magnets, to keep the magnet splinter guard inposition. The clamping strip 5 also serves as a twist preventer toprevent twisting of the magnet splinter guard. As a result, theoverlapping region 4 of the magnet splinter guard 3 can be located on apole lift 10 of the permanent magnet 2, where the thickness of thepermanent magnet 2 decreases continuously down to a predetermined amountA (see FIG. 2). Thus the overlapping region 4 is located radiallyoutside the diameter of the magnet splinter guard, and a constant insidediameter of the magnet splinter guard 3 can be attained. This assuresthat the air gap between the stator and the rotor, not shown, of theelectrical machine will remain constant, or can be reduce, with anincrease in power, in comparison to the prior art.

For centering the magnet splinter guard 3, the cylindrical magnetsplinter guard is centered by means of two centering rings 8, which areeach located on respective ends 6, 7, located in the axial direction, ofthe magnet splinter guard 3 (see FIG. 1). The centering rings 8 have atapering region 9, which centers the magnet splinter guard from itsinside during installation. It is also possible for the magnet splinterguard to be embodied as spread slightly open, so that an additionalholding function can be exerted by the centering rings 8. However, thisspreading open must extend only so far that there is still a constantcoverage in the overlapping region 4, so that any magnet splinters thatmight have splintered off can be prevented with certainty from reachingthe air space between the stator and the rotor. It should also be notedthat the magnet splinter guard 3 is embodied such that in the installedstate, it automatically exerts an axially outward-orientedspreading-open force, so that it presses closely against the magnets 2.

Thus by means of the magnet splinter guard of the invention, it can beassured that no splintered-off particles can get into the air gapbetween the stator and the rotor, where in an extreme case they wouldcause blocking of the electrical machine. The magnet splinter guard 3 isvery simple in construction and can be produced economically and alsoinstalled simply.

FIG. 3 shows a magnet splinter guard in a second exemplary embodiment ofthe present invention. Unlike the first exemplary embodiment, in thisfurther exemplary embodiment, a graduated region 11 is embodied on theoverlapping region 4, so that the inner part of the overlapping regioncan press directly against the step of the graduated region 11. Aconstant inside diameter of the magnet splinter guard 3 is thusattained, so that the small offset at the overlapping region that existsin the first exemplary embodiment is eliminated. To have enough space inthe radial direction outward, the overlapping region 4 is located on thepole lift 10 of the magnet 2 where the thickness of the magnet isreduced continuously down to the amount A (see FIG. 2). Otherwise, thisexemplary embodiment is equivalent to the first exemplary embodiment, sothat the description of the latter may be referred to.

In FIGS. 4 through 7, various possible embodiments of the centeringrings 8 are shown. In the exemplary embodiment shown in FIG. 4, thetapering region 9 is embodied as a cone. A recess 12 is also embodied onthe cylindrical ring 8, in which the magnet splinter guard 3 or moreprecisely an axial end of the magnet splinter guard is located in itsfinal installed position. Slight clamping in the recess 12 may beprovided to make it possible to establish a secure connection betweenthe magnet splinter guard 3 and the centering rings 8. In the exemplaryembodiment shown in FIG. 5, the tapering region is also embodiedconically, but on the end 6 of the magnet splinter guard 3, a radiallyoutward-oriented region 13 is embodied, making easy installationpossible. The exemplary embodiment in FIG. 6 has a tapering region 9which is embodied as an inward-bulging region, and the exemplaryembodiment shown in FIG. 7 has a tapering region 9 which is embodied asan outward-bulging region.

A further advantage of the invention is that besides the centering bymeans of the centering rings 8, any production variations that may existcan also be compensated for. Moreover, in the installed state, thecentering rings 8 can completely close off the space in which themagnets 2 are disposed, so that there is no possibility of magnetsplinters penetrating to the outside.

Moreover, the overlapping region 4 also offers the possibility ofcompensating for temperature changes that occur during operation, sincethe magnet splinter guard 3, because of the overlapping region 4, can bespread apart to a certain extent in the circumferential direction,without lessening the protective function. Nevertheless, a constant airgap between the stator and the rotor can be maintained.

1. An electrical machine, in particular a direct current motor for vehicles, comprising: a multi-pole stator having an annular pole housing (1) and a plurality of magnets (2) that are located on an inside face of the pole housing (1); and a magnet splinter guard (3), wherein said magnet splinter guard (3) shields the magnets (2) inward in the radial direction toward a rotor, wherein the magnet splinter guard (3) is formed from a rectangular blank, has an overlapping region (4), extending in the circumferential direction over an axial length of the magnet splinter guard (3), and on each end (6, 7) of the magnet splinter guard (3) located in the axial direction, a respective centering ring (8) is located, for centering the magnet splinter guard (3), wherein the magnets (2) have pole lift (10), and the overlapping region (4) of the magnet splinter guard (3) is located on the pole lift (10).
 2. The electrical machine according to claim 1, wherein the magnet splinter guard (3), in the installed state, automatically exerts a radially outward-oriented prestressing force on the magnets (2).
 3. The electrical machine according to claim 1, wherein the centering rings (8) each have a tapering region (19).
 4. The electrical machine according to claim 1, wherein the centering rings (8) enclose the magnets (2) between the pole housing (1) and the magnet splinter guard (3).
 5. The electrical machine according to claim 1, further comprising a clamping strip (5), wherein the clamping strip (5) is located on the outer circumference of the magnet splinter guard (3) and in the installed state is located between two magnets (2).
 6. The electrical machine according to claim 1, wherein the axial ends (6, 7) of the magnet splinter guard (3) are slightly bent radially outward.
 7. The electrical machine according to claim 1, wherein the magnet splinter guard (3) at the overlapping region (4) has a graduated region (11), so that the magnet splinter guard (3) in the installed state has a constant Inside diameter.
 8. The electrical machine according to claim 1, wherein the magnet splinter guard (3) is joined to the overlapping region (4) in captive fashion.
 9. The electrical machine according to claim 1, wherein the tapering region (9) of the centering rings (8) is embodied as a cone or as an outward-bulging region, or as an inward-bulging region, or as a stepped tapering region. 